Pumping systems are often deployed into wells to recover petroleum fluids from subterranean reservoirs. Typically, the submersible pumping system includes a number of components, including one or more electric motors coupled to one or more high performance pumps. In the past, large induction motors have been used to drive the pump. The induction, or “squirrel cage,” motors tend to be long. These long motors present deployment problems in certain applications, including deviated wellbores and surface applications with limited space.
The electric motor is often driven by a variable speed drive located on the surface. The variable speed drive produces an alternating current that is transferred to the electric motor through a power cable. In many modern pumping systems, the variable speed drive produces a low voltage, pulse width modulated (PWM) current at a selected frequency. The waveform produced by the variable speed drive can be adjusted manually or automatically to adjust the operating parameters of the pumping system. Step-up transformers can be used to modify the output of the variable speed drive to the design voltage range of the motor.
Recently, motor drives have been provided with have control features called “Field Oriented Control,” or “Vector Control”, that attempt to use the motor voltage and current information to identify motor rotor position. With this position information the drive can commutate the applied voltage in a way that yields better performance and a higher level of control than other open-loop drive techniques. In vector control schemes, the stator currents of the three-phase AC electric motor are identified as two orthogonal components that can be visualized with a vector. One component defines the magnetic flux of the motor (d), the other the torque (q). The control system of the drive calculates from the flux and torque references given by the drive's speed control the corresponding current component references. Vector control can be used to control AC synchronous and induction motors and can be used to operate a motor smoothly over the full speed range, generate full torque at zero speed, and have high dynamic performance including fast acceleration and deceleration.
Although effective, the vector control algorithms are based on calculations using a motor model that is established during manufacture, prior to operation. In applications where the load varies during the service life of the motor, the static vector control algorithm does not always obtain optimal performance from the motor. Additionally, the existing control algorithms are set during manufacture so that the motor operates at a relatively constant efficiency or power output. Once the efficiency of the motor has been established, the power output from the motor is increased by making the motor longer. The inability to adjust efficiency and power output in the field presents a significant drawback in existing systems. There is, therefore, a need for an improved motor control system that is well-suited for use with permanent magnet motors and that provides a greater range of operational characteristics in the field.